In recent years, along with an increase in demand for mobile electronic devices such as a mobile phone or a laptop computer and development of an electric vehicle (EV) and the like, a demand for power storage devices such as an electric double layer capacitor (EDLC), a lithium-ion rechargeable battery (LIB), and a lithium ion capacitor (LIC) has been significantly increased.
The electric double layer capacitor (EDLC) is a capacitor using an electric double layer capacitance; its capacitance is very high among capacitances of capacitors. The electric double layer capacitor (EDLC), which requires no electrochemical reaction, features high stability and high durability such as a long cycle life. However, the energy density of the EDLC as a power storage device is inferior to that of a lead battery, though the EDLC has better characteristics as a capacitor.
In view of the above, a lithium ion capacitor (LIC) has been proposed, which is a hybrid capacitor consisting of a capacitor and a battery, in which graphite where intercalation and deintercalation of lithium ions occur are used for the negative electrode and an electric double layer capacitance is used for the positive electrode. The lithium ion capacitor (LIC) whose negative electrode has a very low potential due to doping (predoping) of lithium to the negative electrode can operate to output a high voltage of about 2 V to 4 V; therefore, the lithium ion capacitor (LIC) has an energy density far higher than that of EDLC.